Bordesley Independent School in Birmingham.
Photograph: Andrew Fox for the Guardian

Fifteen illegal schools educating 800 children – some in squalid conditions and with inadequate safeguarding – have been found to be operating out of sight of local authorities in London, Birmingham and other parts of England.

The unregulated schools were uncovered after a series of tipoffs to the Department for Education and raids by Ofsted inspectors. They appear to have used loopholes involving home study clubs as a way of avoiding regulation, and in some cases had an emphasis on Islamic or Jewish religious study, although some had no obvious religious affiliation.

In the case of one illegal Islamic school uncovered in Bordesley Green, east Birmingham, inspectors found “squalid conditions, including three single mattresses covered in filthy sheets in one room and no running water in the toilet areas”.

Ofsted chief inspector Sir Michael Wilshaw called for the ‘full force of the law’ to be used against illegal schools. Photograph: Andrew Fox for the Guardian

Ofsted’s chief inspector, Sir Michael Wilshaw, called for greater powers for authorities to identify and close unregistered schools. In a letter to the education secretary, Nicky Morgan, he wrote: “I remain concerned that not enough is being done to stop this illegal activity. Too many children remain at significant risk of harm.

“The fact that these so-called schools are unregistered means that they are hidden from the view of local authorities, the Department for Education and Ofsted. It is vital, therefore, that when we do identify such illegal activity, the full force of the law is brought to bear on these institutions to stop them operating without delay.”

So far the DfE has identified 28 potential sites, of which more than half have turned out to be unregistered schools.

Wilshaw accuses local authorities of not acting swiftly enough to target the unregistered schools, and singled out Birmingham city council as particularly at fault.

Inspectors made five visits to the site of the unregistered school in Bordesley Green. It was only after Wilshaw invoked Ofsted’s powers that inspectors gained entry, accompanied by West Midlands police.

Ofsted inspectors reported serious concerns over the welfare of children using the school. “At the most recent visit, inspectors were intentionally obstructed from entering the premises for an hour,” said Wilshaw.

“During this time, inspectors observed a number of female students to be on the premises. When inspectors finally gained entry, the girls had left the building by an alternative exit.”

Staff told inspectors that these girls “were attending a local library and that they would return later in the day”, but they failed to reappear.

Ofsted found timetables suggesting that teaching was going on at the school for at least 20 hours a week, as well as “clear evidence of segregation” with separate classrooms for boys and girls, no evidence of vetting checks carried out on staff, and no information about the whereabouts of children who studied at the school.

In response, Morgan said: “I am extremely concerned by the issues raised by Ofsted. We understand Bordesley has now closed and are keeping in close contact with Birmingham local authority as they work to urgently ensure the children involved are safe and are receiving suitable education. “We are considering Ofsted’s recommendations and will not hesitate to pursue prosecutions where necessary. I welcome Ofsted’s agreement to work closely on this.”

Birmingham city council confirmed that the school, named as Bordesley Independent school, has now closed.

Wilshaw said in his letter: “While our findings in respect of unregistered schools, and particularly Bordesley, are deeply worrying, my regional directors continue to express concern that there are far greater numbers of children hidden away from view in unregistered schools across the country.

“Local authority officers acknowledge the problem, but too often use the excuse of bureaucracy, legislation or lack of resources as a reason for inaction. I remain particularly concerned about the lack of action taken in response to these issues by Birmingham city council.”

Situated on an industrial estate in Bordesley Green, just around the corner from Birmingham City’s St Andrews football stadium, the school’s red-brick building shows little sign it was ever open beyond a Bordesley Independent sign above the main reception.

It has no playground or fields, with only a secure car park at the front. It is surrounded by three steelworks and countless frozen food companies, all housed in separate industrial units.

Two women who were at the building at school closing time on Tuesday said they believed Bordesley Independent closed “suddenly” very recently. Two former pupils turned up earlier in the day wanting to collect their schoolbooks, they said, while the sister of another pupil was shocked to find it closed down. “I think even the parents weren’t officially told,” one of the women said.

Inside, the shutters were down on the school’s reception and the adjoining ground-floor room almost completely empty, save for a table, a prayer mat and books brought by women who on Sunday began using the room as a social space. The floor’s carpet was dirty when they visited for the first time, one woman said.“Unregistered? It’s been here about two years, maybe more,” said her friend, who also did not want to be named.

A man inside the building, who asked not to be named, said the building housed Arabic classes for children at 4pm on some weekdays, although it is not clear whether these classes were part of the school. “There were 10 or 15 students who did their exams from here because this is a GCSE exam centre as well,” he said.

Naveed Hussain, 37, is named on Companies House documents as the sole director of the school, which appears to have been incorporated last October. Hussain, a teacher and local businessman, is also registered as jointly owning a nursery called Clever Kloggs in nearby Small Heath.

The nursery showed no sign of being open on Tuesday afternoon, although its front had been decorated with a colourful mural saying “play, learn and grow together”. The nursery was only registered at Companies House in September, documents show. The shopkeeper next door said the nursery had not yet opened.

The Guardian was unable to reach Hussain by telephone or at his home address on Tuesday.

A 10-minute walk from the now-closed Bordesley Independent sits Bordesley Green girls’ school, which was given an overall rating of “outstanding” in its latest Ofsted inspection in March 2015.

A short walk away is the Al-Hijrah school, which was placed in special measures by Ofsted in December 2013 and reported earlier this year to be £3m in debt. Birmingham city council, which funds the voluntary-aided Islamic school, was investigating the Islamic school’sits spending and helping it out of its financial mess.

Birmingham council rejected Wilshaw’s comments, saying that it was “unacceptable” for children to be taught in the illegal institutions.

“Here in Birmingham we raised our concerns with Ofsted and the Department for Education several months ago and are glad that Ofsted is now taking action,” said Brigid Jones, the council’s cabinet member for children’s services.

“The DfE now has to play its part, act to strengthen the law, and use its powers to find these places, close them down and prosecute those responsible.”

Wilshaw said that the DfE’s arrangements for safeguarding children in unregistered schools needed to be urgently reviewed and home schooling regulations examined to ensure they cannot be exploited by illegal school operators.

“From conversations with some of the children and young people my inspectors spoke to, there is evidence to suggest some of these schools are using the freedoms afforded to genuine home educators as a cover for their activities,” he noted.

By law, private schools must be registered and inspected, although few regulations apply to “home study groups” or private tuition clubs. David Cameron raised the issue of regulating so-called “supplementary schools” during his speech to the Conservative party conference last month.

“Since 2010 we have taken robust steps to tackle unregistered schools and improve safeguarding. However we agree with Sir Michael that more needs to be done,” Morgan said in a statement.

“We will introduce further powers to regulate settings which teach children intensively and to intervene and impose sanctions where there are safety or welfare concerns. We will be consulting on these proposals shortly.”